The paper industry uses additives to improve the properties of paper. Additives may be added to paper pulp slurry prior to sheet formation (wet-end addition), or applied to paper after sheet formation (dry-end addition). Some properties that are desired in paper depending on its application include wet strength, dry strength, folding endurance, oil resistance and solvent/stain resistance.
For ease of processing and commercial applicability, the curing temperature of the paper should be as low as possible, the time of treatment should be short, and the cost of the additives used should be low. Also, the treatments should not emit environmentally harmful products. Additives currently used to improve wet strength, such as urea-formaldehyde (UF), melamine-formaldehyde (MF), and polyamide/polyamide-epichlorohydrin (PAE) are believed to emit formaldehyde (UF and MF) or absorbable organic halogens (AOX).
In an attempt to produce paper with desired properties, as well as decrease the amount of environmentally harmful chemicals being produced from paper production, various additives have been studied.
The dry properties of the paper are affected by the nature of crosslinks produced in the paper network. Small molecules such as glutaraldehyde are able to penetrate readily into cellulosic fibers and produce crosslinks between cellulosic molecules inside the fiber (Linke, W. F., (1968) Tappi J. 51(11):59A-65A). These crosslinks are located predominantly in the amorphous regions of the fiber wall and restrict the mobility of microstructural units of fibers. The intrafiber crosslinks produced by the small molecules such as glutaraldehyde alone do not contribute to fiber bonding and thus have little effect on the dry strength of paper. However, the intrafiber crosslinks increase the rigidity of fibers, and thus impart a more heterogeneous distribution of stress in the paper network. The result of intrafiber crosslinks is the reduction in the stretch of treated paper. The decrease in the stretching ability of paper also leads to premature breakage of paper, thus leading to an overall reduction in dry strength. Paper treated with high levels of glutaraldehyde exhibits loss of dry strength and decreased folding endurance.
Certain dialdehydes have been studied as crosslinking agents of cellulose to impart wrinkle resistance to cotton fabric (Frick, J. G., et al., (1982) J. Appl. Polym. Sci. 27:983-988; Frick, J. G., et al. (1984) J. Appl. Polym. Sci. 29:1433-1447). The reaction between certain dialdehydes and cellulose is promoted by using metallic salts and ammonium salts as catalysts (Petersen, H. A. (1983) in Chemical Processing of Fibers and Fabrics: Functional Finishes Part Axe2x80x9d, Chapter 2, Lewin, M et al. (eds.), New York, pp. 47-327). The most frequently used metallic catalysts include inorganic salts of aluminum, magnesium and zinc.
Glyoxal has been used to provide temporary wet strength for paper (Eldred, N. R. et al. (1963) Tappi 46(10):608-612). Glyoxal crosslinks cellulose molecules by formation of hemiacetal links, which are sensitive to water. If an acid catalyst is used, more stable acetal linkages are reported to be formed. However, the use of glyoxal, especially in an acidic treatment process, leads to serious embrittlement of paper, similar to the use of small polycarboxylic acids.
U.S. Pat. No. 4,547,807 (Chan et al.) relates to a method of temporarily improving the wet strength of pre-moistened towelette paper by reacting the paper with an adhesive formed from the reaction of a polyvinyl alcohol with glyoxal or related dialdehydes. Papers produced by the disclosed process reportedly exhibit enhanced wet strength when stored in an acidic medium and decreased wet strength when immersed in a basic medium. No catalyst is used.
U.S. Pat. No. 4,888,093 (Dean et al.) reports the use of certain dialdehydes to reportedly provide individualized, crosslinked cellulosic fibers.
Fully hydrolyzed polyvinyl alcohol (PVA) is a polymer with high tensile strength, excellent flexibility, good water resistance, and outstanding binding capacity (Finch, C. A. Ed., xe2x80x9cPolyvinyl Alcohol: Properties and Applications,xe2x80x9d John Wiley and Sons, (1973), pp.277-230). Polyvinyl alcohol (PVA) has been used in the paper processing industry for surface and internal sizing of paper and to impart water resistance to paper.
Large amounts of PVA (approximately 10% on an oven-dried basis of the pulp) have been added to pulp in wet-end processing to reportedly enhance paper wet strength (U.S. Pat. No. 2,402,469 (Toland et al.), discussed in U.S. Pat. No. 5,328,567 (Kinsley)). The PVA product described is water soluble at 130xc2x0 F. PVA has been reported to control pitch deposition on processing equipment used when paper is formed (U.S. Pat. No. 4,871,424 (Dreisbach et al.)). PVA has also been reported to control xe2x80x9cstickiesxe2x80x9d that may form during paper processing from the adhesives, ink and coating binders remaining from papers used in the recycled paper industry (U.S. Pat. No. 4,886,575 (Moreland)).
U.S. Pat. No. 5,281,307 (Smigo et al.) discloses the use of polyvinyl alcohol/vinylamine copolymers with a crosslinking agent to treat paper in a dry-end process. No catalysts are used. The process disclosed required a heat treatment of 150xc2x0 C. for 5 minutes after the paper was formed.
U.S. Pat. No. 5,380,403 (Robeson et al.) discloses the use of an amine functionalized polyvinyl alcohol in combination with a cyclic ester or anhydride in a wet-end process to reportedly improve the wet strength of recycled paper.
Even though many additives have been studied in an attempt to improve the properties of paper, there is a continuing need for an environmentally-friendly treatment that imparts permanent improvement in wet strength, dry strength and folding endurance to the paper.
All references cited herein are hereby incorporated by reference to the extent not inconsistent with the disclosure herein.
A method of treating paper with a hydroxy-containing polymer and a multifunctional aldehyde in the presence of a catalyst to improve the strength properties of paper including wet strength, dry strength and folding endurance is provided. As used herein, xe2x80x9cpaperxe2x80x9d includes paper sheets after formation, or paper pulp before sheet formation, among other forms as known in the art. In the method of the invention, the aldehyde is present in amounts of between about 50% to 800% weight percent of the polymer present, and all intermediate ranges therein. Preferably, the aldehyde is used in an amount of greater than 100% the weight percent of polymer. The catalyst is preferably present at a percentage weight ratio between about 1:0.1 to about 1:2 aldehyde:catalyst and all intermediate ranges therein, and more preferably present at a weight ratio of 1:0.2 to 1:1 aldehyde:catalyst. The treatment chemicals are typically used in total amounts of between about 0.1% to about 10% based on dry weight of pulp fibers, and all intermediate ranges therein. The presently preferred treatment uses between about 0.25% to about 4% by weight of treatment chemicals, based on dry weight of pulp fibers.
Also provided is a method of treating paper comprising: contacting said paper with a non amine-functionalized hydroxy-containing polymer and a multifunctional aldehyde, wherein the multifunctional aldehyde has formula: 
where R is a divalent aliphatic, cycloaliphatic, aromatic or heterocyclic group having from 1 to 12 carbon atoms; the multifunctional aldehyde is present at a concentration of about 50% to about 800% weight percent of the polymer present; and the total weight of hydroxy-containing polymer and multifunctional aldehyde is about 0.1% to about 10% based on the dry weight of pulp fibers. The hydroxy-containing polymer and multifunctional aldehyde composition may further comprise a catalyst selected from the group consisting of AlCl3, Al2(SO4)3, Al(NO3)3, alum, ZnCl2, Zn(NO3)2, Zn(CH3COO)2, MgCl2, Mg(NO3)2, Mg(CH3COO)2, NH4Cl and amino acids; wherein said catalyst is present at a weight ratio between about 1:0.1 to about 1:2 multifunctional aldehyde:catalyst; and the total weight of hydroxy-containing polymer, multifunctional aldehyde and catalyst is about 0.1% to about 10% based on the dry weight of pulp fibers.
Contacting means providing an appropriate vessel or apparatus so that the treatment chemicals are in contact with the paper. Method of contacting are known in the art.
Preferably, the multifunctional aldehyde is gluteraldehyde and said hydroxy-containing polymer is non-amine functionalized poly vinyl alcohol. Preferably, the multifunctional aldehyde:polymer weight ratio is greater than 1:1. Preferably the catalyst is present at a weight ratio from about 1:0.2 to about 1:1 multifunctional aldehyde:catalyst. It is preferred that the total weight of hydroxy-containing polymer, multifunctional aldehyde and catalyst is between about 0.25% to about 4% by weight of the dry weight of paper treated.
The method may further comprise curing the paper at a sufficient conditions to cause the desired improvement in strength properties of said paper. These conditions are well known in the art without undue experimentation using the knowledge in the art and the teachings described herein. One example of such conditions is a temperature which is between about 100 to about 150xc2x0 C. for a time of between about 0.5 and about 5 minutes. Other temperatures and times may be used, for example all intermediate temperature ranges and times therein.
Also described is paper treated with the methods described herein.
Also provided is a paper treatment composition comprising: a non amine-functionalized hydroxy-containing polymer and a multifunctional aldehyde wherein the multifunctional aldehyde has formula: 
where R is a divalent aliphatic, cycloaliphatic, aromatic or heterocyclic group having from 1 to 12 carbon atoms; the multifunctional aldehyde is present at a concentration of about 50% to about 800% weight percent of the polymer present; and the total weight of hydroxy-containing polymer and multifunctional aldehyde is about 0.1% to about 10% based on the dry weight of pulp fibers.
In this composition, it is preferred that the multifunctional aldehyde is gluteraldehyde and said non amine-functionalized hydroxy-containing polymer is PVA. This composition may further comprise a catalyst selected from the group consisting of: AlCl3, Al2(SO4)3, Al(NO3)3, alum, ZnCl2, Zn(NO3)2, Zn(CH3COO)2, MgCl2, Mg(NO3)2, Mg(CH3COO)2, NH4Cl and amino acids, wherein the catalyst is present at a weight ratio between about 1:0.1 to about 1:2 multifunctional aldehyde:catalyst; and the total weight of hydroxy-containing polymer, multifunctional aldehyde and catalyst is about 0.1% to about 10% based on the dry weight of pulp fibers.
Also provided is a method of using the composition described above in a paper making process comprising: contacting said paper with the composition described above for a time sufficient to deposit the desired amount of composition on said paper; curing said paper at a sufficient conditions to cause the desired improvement in strength properties of said paper. These conditions are well known to one of ordinary skill in the art without undue experimentation with the teachings described herein. One example of sufficient conditions to cause the desired improvement in strength properties of said paper is a temperature which is between about 100xc2x0 C. to about 200xc2x0 C. for a time which is between about 0.5 to about 5 minutes.
Also provided is an improvement for a papermaking process producing paper having improved strength properties, said improvement comprising: adding a non amine-functionalized hydroxy-containing polymer, a multifunctional aldehyde and a catalyst to paper wherein: the catalyst is selected from the group consisting of: AlCl3, Al2(SO4)3, Al(NO3)3, alum, ZnCl2, Zn(NO3)2, Zn(CH3COO)2, MgCl2, Mg(NO3)2, Mg(CH3COO)2, NH4Cl and amino acid; the multifunctional aldehyde has formula: 
where R is a divalent aliphatic, cycloaliphatic, aromatic or heterocyclic group having from 1 to 12 carbon atoms; the multifunctional aldehyde is present at a concentration of about 50% to about 800% weight percent of the polymer present; the catalyst is present at a weight ratio between about 1:0.1 to about 1:2 multifunctional aldehyde:catalyst; and the total weight of hydroxy-containing polymer, multifunctional aldehyde and catalyst is about 0.1% to about 10% based on the dry weight of pulp fibers.
This process may further comprise curing the paper for a sufficient time and temperature to cause the desired improvement in properties to occur. These conditions are preferably a temperature which is between about 100 to about 500xc2x0 C. and a time which is between about 0.5 to about 5 minutes.
As used herein, xe2x80x9cmultifunctional aldehydexe2x80x9d is an aldehyde compound with 2 or more aldehyde functionalities. Multifunctional aldehydes that may be used in the method of this invention include dialdehydes of general formula: 
where R is a divalent aliphatic, cycloaliphatic, aromatic, or heterocyclic moiety and mixtures thereof. Preferred dialdehydes include aliphatic dialdehydes where R is a divalent aliphatic hydrocarbon moiety having 1 to 12 carbon atoms, and all intermediate ranges therein, more preferably from 1 to 6 carbon atoms, such as glutaraldehyde, furan dialdehydes, 2-hydroxyadipaldehyde, and succinaldehyde. Polymeric multifunctional aldehydes, such as dialdehyde starch, polyacrolein, poly(meth)acrolein, copolymers of acrolein and methacrolein, and their derivatives may be used in the methods of this invention. Hemiacetal or acetyl compounds that produce aldehyde groups during the application process can also be used. The most preferred multifunctional aldehyde is glutaraldehyde. Mixtures of multifunctional aldehydes may be used in this invention.
Catalysts that promote the reaction between dialdehydes and cellulose and that may also improve the efficiency of the crosslinking system that can be used in the invention include metal salts, such as AlCl3, Al2(SO4)3, Al(NO3)3, alum, ZnCl2, Zn(NO3)2, Zn(CH3COO)2, MgCl2, Mg(NO3)2, Mg(CH3COO)2. The catalysts also include NH4Cl and amino acids. The salt catalysts may be used in combination with hydroxyl-containing carboxylic acid, such as citric acid or tartaric acid. Acid catalysts are believed to form more stable acetal linkages. The process may be used along with other additives such as other crosslinkers, defoamers, biocides, plasticizers, among others, either in wet-end or dry-end processing. Other additives that may be used include multifunctional carboxylic acids, specifically poly(maleic acid), which is believed to crosslink cellulose.
xe2x80x9cHydroxy-containing polymersxe2x80x9d used in this invention may be derivatized. More preferred hydroxy-containing polymers of this invention include polyvinyl alcohol (PVA) and its derivatives including homo- and copolymers of vinyl alcohol, such as poly(ethylene-vinyl alcohol), completely and partially hydrolyzed PVA. Other PVA derivatives that may be used include cationic PVA copolymers with cationic acrylamide monomers, such as dimethylaminoethyl methacrylate dimethylsulfate quaternary, and cationic PVA formed by reaction of PVA with cationizing agents, such as 2,3-epoxypropyl-N, N, N, -trimethylammonium chloride. PVAs also include various denatured PVA, such as carboxyl or amino denatured PVA. The degree of hydrolysis is not particularly limited, with high and superhydrolyzed PVA preferred. PVA with Brookfield viscosity larger than 5 centipoises and not less than 20 centipoises (when measured at 20xc2x0 C. with 4% aqueous solutions) is preferred. Preferably the polymer is not amine functionalized.
As used herein, xe2x80x9cpaperxe2x80x9d includes paper products both before sheet formation (paper pulp) and after sheet formation. All grades of paper products, including paperboards, carton and corrugating medium, cellulosic nonwovens, wood fiber-nonwood fiber combinations, and other grades of paper products as known in the art are included within the scope of the term.
Also provided are compositions of polyvinylalcohol, multifunctional aldehyde, preferably gluteraldehyde and catalysts that may be used in the methods of this invention as paper additives to improve the properties of paper. Additionally, paper or paper pulp treated with compositions of polyvinylalcohol, glutaraldehyde and catalysts as described in this invention are provided.
The invention provides advantages over current technology: being environmentally safe without emission of formaldehyde or AOX; improving both wet and dry strength; improving paper wet strength while increasing folding endurance, providing permanent wet strength improvement; using low curing temperatures and shorter curing time compatible with current papermaking machines; reasonable cost; and good repulpability of the resulting paper. The use of a process that imparts both wet and dry strength means that lower grades of pulp fibers and recycled pulp fibers can be utilized to a greater extent.
Applicant does not wish to be bound by any particular theory but, it is believed that when a hydroxy-containing polymer such as PVA is added to a multifunctional aldehyde, the aldehyde will readily react with hydroxyl groups in the polymeric chains through the formation of hemiacetal or acetal links. Thus, the polymer is believed able to chain the aldehyde molecules together. For example, when PVA and glutaraldehyde are used, it is likely that pentanedialated-PVA is formed. Pentanedialated PVA is illustrated in Scheme 1. Scheme 1 shows unreacted hydroxyl groups of PVA, reactive free aldehyde, and hemiacetal groups. If the concentration and ratio between PVA and glutaraldehyde are appropriately controlled, the pentanedialated-PVA is believed to remain sufficiently reactive and stable enough to react with cellulose molecules and produce crosslinks. Because of the bulky size of high molecular weight PVA, it is believed that the pentanedialated-PVA does not penetrate into the fiber interior but stays on the fiber surface, moves to the fiber crossing area, and is driven into pores and cracks in the fiber surface by capillary forces. Long-range crosslinks between fibers are believed to be produced. It is also possible that free glutaraldehyde is produced.
Pentanedialated-PVA may improve the dry performance of paper in two ways. First, the interfiber crosslinks may reinforce the fiber bonding and thus increase the dry strength. Second, since the interfiber area is the center of stress transfer in the network, and the pores and cracks are the weak points of the fiber, presence of polymer may strengthen the weak points of fibers and enhance the stress dissipation of network. The aldehyde may also react with cellulose, producing interfiber and heterogeneous crosslinking. Thus, crosslinking using a polymer/aldehyde system is more effective to improve the wet performance of paper without sacrificing dry properties than a system using an aldehyde alone.